The present apparatus relates broadly to a microwave filter apparatus, and in particular to a dual mode directionally coupled band reject filter apparatus.
Microwave band rejection filters have been generally defined as combinations of resonant and antiresonant circuits connected to either transmission lines or waveguides so that the undesired band of frequencies is selectively attenuated from the total frequency spectrum. In the case of waveguide filters, the resonant circuits take the physical form of resonant cavities which are coupled to the waveguide by means of an opening or iris in the wall of the waveguide. The specific cross-sectional shape of the opening (round, square, rectangular, etc.) permits certain propagation modes of R-F energy to pass from the waveguide into the cavity thereby providing the excitation energy sufficient to propagate various modes within the cavity. The openings or irises which are associated with a given cavity are situated in odd multiples of .lambda..sub.g/ 4, where .lambda..sub.g refers to the main waveguide wavelength. A quarter wavelength opening spacing provides an energy loss or attenuation over a particular spectral range due to the properly phased wave-cancellation which is caused by the resonant behavior of the cavities.
The amount of attenuation of waveguide energy at any given frequency is determined by the shape and positioning of the coupling opening, the dimensions and form of the cavity and the number of cavities employed in a given band rejection filter configuration. Attenuation characteristics may be predicted by utilizing lumped element prototype filter models which are well known in the art of conventional network synthesis techniques.
In the prior art, band reject waveguide filters which provide microwave band reject filtering have used rectangular or cylindrical cavities that are spatially situated along a straight section of waveguide such that each cavity will support a single mode of propagation. The openings which are associated with each such cavity have been located at a distance which is some multiple of a quarter wavelength of the waveguide signal along the length of a straight waveguide section. However, such configurations in the prior art have had the disadvantage of requiring large physical size, being physical complexity and incurring high fabrication costs.
Multicavity dual-mode band pass waveguide filters which utilize two or more cavities are well known in the prior art. Such waveguide filters provide two or more cavities each of which cavity resonates in two orthogonal modes and are coupled together through an iris or opening in a correct sequence. The various methods of coupling the respective cavities to the waveguide will provide desired response characteristics. The coupling between the modes in each of the cavities is provided by a structural discontinuity or obstacle which is placed in the cavity. The resonant frequency of each mode within each cavity may be adjusted by a respective tuning screw. In this way for a two cavity system, there are required two tuning screws in each of the cavities.
In such prior systems, the input is applied to the circular cross sectional end of one cavity and the output is taken from the opposing circular cross sectional end of the last cavity. Coupling between the cavities is provided by an iris positioned in a circular cross sectional separating each pair of cavities. However, these filters are band pass, not band reject and the modes in each cavity may be tuned by tuning screws positioned therein. One type of prior art arrangement utilizes a multicavity waveguide filter in which each of the cavities has a pair of orthogonally related modes of propagation. The cavities are positioned side by side with the long axis of each of the cavities being aligned parallel to each other. The cavities are tuned by tuning plungers which are simultaneously moved to predetermined positions within the cavity. Waveguide filters of this type are cumbersome to operate.